The present invention relates to electroplating gravure rolls with a surface layer of copper. More particularly, it concerns the use of a unique plating bath formulation which results in a surface coating that is ideally suited for electronic engraving.
Gravure printing is a method using the Intaglio process in which the image to be printed consists of depressions etched or engraved usually to different depths. Slightly viscous solvent inks are applied to the entire surface and a metal doctor blade removes the excess ink from the non-printing surface. Normally engraving is performed on a copper plated cylinder which is subsequently chrome plated to minimize wear.
A problem in gravure cylinder making is the difficulty in producing cylinders having surface properties which are identical from cylinder to cylinder. Surface defects such as roughness, pits or spots which are too hard or too soft result in engraving errors and the subsequent need for repolishing and replating which is expensive and time consuming.
The properties of the plated copper deposits have not been considered important for this application when chemical etching has been the engraving procedure. However, since the development of the automatic method of electronic engraving, the electrodeposition of copper of known physical and mechanical properties with reproducible grain size, crystal structure and hardness over the entire surface of the roll has become very important.
Prior art acid copper plating processes are primarily directed toward decorative plating where the objective is to impart leveling and brightness characteristics with little regard to the precise physical properties that are so important for electronic engraving. Such decorative applications are generally concerned with deposits ranging in thickness from about 0.0005 to about 0.0015 in. while gravure rolls require deposits ranging from 10 to 20 times these thickness values.
Copper plating applied to gravure cylinders from processes designed for decorative applications tends to produce grain structures and hardness values not suitable for electronic engraving. These copper deposits may show the initial requisite hardness value but within a short period of time, undergo spontaneous structural changes (often referred to as annealing) so that the deposit is too soft for commercially acceptable electronic engraving.
Attempts to overcome the annealing problem with higher concentrations of the constituents typically used for decorative acid copper plating, such as thiourea and mercapto compounds, result in deposits that are too hard and brittle.
Electronic engraving is a means of transferring an image for printing to a copper electroplated cylinder by directing a diamond-pointed stylus to form as many as 4,000 ink-receiving impressions every second. This sophisticated technique requires copper deposits of very definite properties to prevent engraving defects and costly damage to the expensive equipment. It is essential that the deposited copper have a homogeneous fine-grained crystal structure that is free of nodulations and occlusions with excellent ductility and uniform hardness. A critical factor is the control and uniformity of hardness since the stylus pressures are set with references to a given Vickers hardness value and if this is not uniform over the entire surface, it will result in smearing or ripping of the deposit and badly defined impressions for printing.
In order to produce the required thick deposits in a reasonable plating time, current densities must be employed in the range of 100 to 200 amperes per square foot or higher, much higher than decorative acid copper plating which is usually accomplished at 25 to 50 amperes per square foot.
The gravure cylinders may be plated either partially or fully submerged, the deposition rate being related to the immersion depth. An acid copper process which has been used to plate partially immersed cylinders is disclosed in U.S. Pat. No. 4,334,966. An important advantage realized by increasing the immersion depth is a decrease in plating time which has obvious economic advantages.
When a cylinder is plated partially immersed, i.e. to about 30% of its diameter, as compared to a cylinder that is plated totally submerged, the deposit characteristics are apparently influenced by the fluctuations of the current and composition differences in the cathode film. In any event, plating baths are known to perform differently with respect to the immersion depth. The principal problem in this regard is annealing or the tendency of the hardness of the copper deposit to decrease with time as a result of changes in crystallite size, texture, microdeformations and dislocations within the copper deposit. This problem of recrystallization (annealing) is characteristic of totally submerged cylinder operations when using a bath designed for partial immersion.
The comparative success of the different processes has been related to the engravability of the deposit as well as the ease of the operation and control of the plating bath. Thiourea has been used in additive systems to overcome the annealing problem experienced when plating fully submerged cylinders. However, such systems require the plating bath to be so nearly free of chloride ions that especially pure reagents and deionized water are needed. Moreover, these systems are prone to inclusions, e.g., hard radial structures localized within the deposit, and generally lack the desired uniformity necessary for quality engraving.
There remains, therefore, a need for an acid copper plating process which can be used to deposit a copper layer of uniform hardness and stability, which is suitable for electronic engraving, on rolls which are plated while completely or nearly completely submerged in the plating bath.